1. Field of the Invention
The present invention relates to a method for measuring an X-ray reflectance in a short time with the use of an X-ray detector having a high upper-limit counting rate, and a method for measuring an X-ray reflectance with a high dynamic range with the use of an X-ray detector having a low noise level.
2. Description of the Related Art
It is known to measure an X-ray reflectance of a thin film and analyze the properties of the thin film, e.g., a film thickness, a density, a surface roughness and a boundary roughness, based on the measured data, as disclosed in Japanese patent publication No. 10-38821 A (1998), which will be referred to as the first publication.
An X-ray reflectance curve can be determined in a manner that a reflected X-ray intensity is measured with the scanning of the scattering angle 2θ in a range between zero to five degrees for instance. If a reflected X-ray intensity is measured at 0.01-degree intervals in 2θ and a measuring time length per interval is set to be one second, there is required, for the range between zero to five degrees, five-hundred measurement steps, the total time required for the measurement being five-hundred seconds. As just described, it takes ordinarily around several minutes to determine one reflectance curve. An X-ray detector may be, for instance, a proportional counter or a scintillation counter.
Incidentally, the present invention is concerned with the use of an avalanche photo diode as an X-ray detector, the avalanche photo diode being abbreviated to “APD” hereinafter. An X-ray analytical apparatus using the APD as an X-ray detector is known and disclosed in, for instance, Japanese patent publication No. 2004-37360 A, which will be referred to as the second publication.
The second publication discloses that a fluorescent X-ray holography detection system uses the APD having a high counting rate so as to complete the measurement in a shorter time than before. It says in the second publication that since the counting rate of the APD is high as not less than 106 cps, the time required for detecting an X-ray at any measuring point is not more than one second and the total measuring time is reduced from around two months to around several hours when the fluorescent X-ray intensity is measured even under many angular conditions.
It is sometimes the case, as in the process of semiconductor device manufacturing, that the X-ray reflectance measurement must be carried out for many measuring positions on a single substrate to analyze the properties of the thin film, such as a film thickness for instance, for respective measuring positions. Assuming that it takes several minutes for the measurement at one measuring position, it takes several dozen minutes for the measurements at ten measuring positions. If the time required for the measurement at one measuring position is reduced to several seconds, it would take mere several dozen seconds for the measurements at ten measuring positions, completing the total measurement process in a very short time. Such a short-time measurement has been desired.
On the other hand, in the field of the X-ray analysis, it is known to use the APD having a high counting rate to reduce the measuring time as disclosed in the second publication. The second publication is, however, concerned with the special use as detecting very small fluorescent X-ray hologram signal, in which the long-term measuring time of around two months with the use of the ordinary detector has been reduced to the several hours with the use of the APD. It is noted that, even using the APD, there is needed around one second for one step of the X-ray intensity measurement in the fluorescent X-ray hologram. On the contrary, in a measurement process in which the total process is completed in around several minutes with the use of the ordinary detector, such as the X-ray reflectance measurement, one measurement step at one scattering angle 2θ is completed in a short time as around one second inherently. It has been unknown to attempt to make such a measuring time further shorter. Besides, since the X-ray reflectance measurement often requires a five-digit dynamic range or over, it would be necessary to insert or remove an absorption plate in the middle of the measurement operation in the case of using the ordinary proportional counter or the scintillation counter having an upper-limit counting rate of not more than 106 cps, the insertion or removable of the absorption plate being an obstacle to the reduction of the measuring time.
On the contrary, it is sometimes desired to measure a high-accuracy X-ray reflectance curve with a high dynamic range. Since the ordinary method uses the proportional counter or the scintillation counter as an X-ray detector, it is impossible to measure a weak X-ray intensity because of the inherent noise level of the X-ray detector and thus it is impossible to carry out the reflectance measurement with a high dynamic range.